Thermal and compositional structure of the subcontinental lithospheric mantle: Derivation from shear wave seismic tomography

Abstract

[1] Seismic tomography can provide unique information on the structure of the subcontinental lithospheric mantle (SCLM), but seismic velocity reflects both temperature and composition. We present a methodology for evaluating and isolating the relative contributions of these effects, which produces maps of regional geotherm and broad compositional constraints on the SCLM from the inversion of shear wave (Vs) seismic tomography. This approach uses model geotherms quantized in steps of 2.5 mW/m 2 and three mantle compositions corresponding to typical Archean, Proterozoic, and Phanerozoic SCLM. Starting from an assumed composition for a volume of SCLM, lithospheric density at surface pressure and temperature is calculated for each geotherm at each point; the optimum geotherm is taken as the one yielding a density closest to the mean value derived from mantle xenoliths (3.31 g/cm 3 ), since density varies with composition. Results requiring densities or geotherms outside the known natural range of these parameters worldwide require the choice of a different mantle composition. This technique, applied iteratively to a 275 km  275 km Vs model developed by S. Grand (University of Texas, Austin), results in maps of the geotherm and regional density, which allow interpretation of SCLM composition within broad limits. These results can G 3

Upper mantle structure of the Saharan Metacraton

“…2B and 3) similar to those of Ritsema and van Heijst (2000), Shapiro and Ritzwoller (2002), Deen et al (2006), Pasyanos and Nyblade (2007), and Begg et al (2009) models. However, the faster S-wave velocity anomalies observed at 0-100 km depth under the Saharan Metacraton decrease sharply to nearly zero at 100-175 km depth (Fig.…”

“…The Ritsema and van Heijst (2000) model shows that the faster S-wave velocity under Egypt persists down to a depth of 150 km, but this disappears at 200 km depth where much of the Saharan Metacraton is shown as been underlain by velocities equal to PREM. Additionally, the Saharan Metacraton was imaged as underlain by a slightly higher velocity relative to PREM at 100-175 km in Deen et al (2006) using S-wave velocity model derived from Grand (2002). The velocity values were not specified in the presentation of Deen et al (2006), but it can be concluded that these are not as high as those under other African cratons.…”

“…In their synthesis, Abdelsalam et al (2002) agreed that the Saharan Metacraton has been destabilized at the end of the Neoproterozoic era, but the metacraton has acted as a coherent rheological entity, hence cannot simply be considered as an orogenic belt with remnants of older reworked components. Seismic tomography studies of global scale (Grand, 2002;Shapiro and Ritzwoller, 2002;Lebedev et al, 2009;Pasyanos, 2010), Africa-wide (Ritsema and van Heijst, 2000;King and Ritsema, 2000;Deen et al, 2006;Pasyanos and Nyblade, 2007;Begg et al, 2009), and those focused on parts of Africa (James et al, 2001;Priestley et al, 2006) have been successful in imaging the West African, Congo, and Kalahari Cratons as blocks with welldeveloped cratonic roots that extend down to a depth of 250 km. However, these studies, with the exception of Deen et al (2006), Begg et al (2009), and Pasyanos (2010) did not directly address the state of the sub-continental lithospheric mantle (SCLM) beneath the Saharan Metacraton.…”

“…Seismic tomography studies of global scale (Grand, 2002;Shapiro and Ritzwoller, 2002;Lebedev et al, 2009;Pasyanos, 2010), Africa-wide (Ritsema and van Heijst, 2000;King and Ritsema, 2000;Deen et al, 2006;Pasyanos and Nyblade, 2007;Begg et al, 2009), and those focused on parts of Africa (James et al, 2001;Priestley et al, 2006) have been successful in imaging the West African, Congo, and Kalahari Cratons as blocks with welldeveloped cratonic roots that extend down to a depth of 250 km. However, these studies, with the exception of Deen et al (2006), Begg et al (2009), and Pasyanos (2010) did not directly address the state of the sub-continental lithospheric mantle (SCLM) beneath the Saharan Metacraton. Here, we use the term SCLM to refer to the chemically depleted rigid lower part of the continental plate that moves with the plate which is isolated from the convective mantle (hence resists homogenization), and the base of it coincides with the low velocity zone (O'Reilly et al, 2001).…”

Geophysical and petrological constraints on ocean plate dynamics

“…In Africa, which in many reconstructions is put as the heart of Greater Gondwana [Irving and Irving, 1982;Smith and Hallam, 1970], surface geological mapping, geochemical and geochronological studies together with low resolution seismic tomography [Abdelsalam et al, 2011;Begg et al, 2009;Deen et al, 2006;Lebedev et al, 2009;Pasyanos, 2010;Pasyanos and Nyblade, 2007;Ritsema and van Heijst, 2000;Shapiro and Ritzwoller, 2002] have resulted in establishing the extent of major cratons and orogenic belts of Greater Gondwana (Fig. 1A).…”